Combined effect of UV-B and heat shock on photosynthetic apparatus of wild type and UV -B-HS-tolerant (UV -HS^t) mutant strain of cyanobacterium <i>Anacystis nidulans</i>

Abstract

Ultraviolet-B (UV-B) and heat shock (HS) induced changes in the photosynthetic apparatus have been studied in the wild type <i>A. nidulans </i>and its ultraviolet-heat shock tolerant (UV-HS^t) mutant strain. Application of the UV -B and HS stresses either separately or in combination showed a drastic photobleaching and alteration in the ratio of phycocyan in and chlorophyll <i>a</i> (PC/Chl <i>a) </i>in the wild type strain as compared to its UV-HS^t mutant counterpart. Both wild type and UV-HS^t mutant strain exhibited similar rate of photosynthetic O₂ evolution at fluorescent high light irradiance (30 Wm^-2) , however, the photosynthetic yield in UV-HS^t mutant strain was 28 fold higher than that of its wild type counterpart. Further, the wild type strain showed UV-B/HS dose-dependent inhibition in the photosynthetic electron transport chain and maximum inhibition was recorded at 5 Wm^-2 of UV -B and 40°C of heat shock. In contrast, UV -HS^t mutant strain showed slight enhancement in its photosynthetic activities at 1 Wm^-2 of UV-B or 35°C of HS and after that followed a decreasing pattern. The ratio of PS II/PS I in wild type strain

increased significantly with dose-dependent increment in UV -B or heat-shock, whereas such changes were found to be insignificant in the UV-HS^t mutant strain. Furthermore, the data ascertained the role of different temperature in recovery of UV-HS- induced damage in presence and absence of light as the high temperature (35°C) contributed the maximum recovery of Hill activity in UV-HS^t mutant strain, whereas, the wild type strain recovered at low temperature (25°C).